5 Selecting and Interpreting Investigations

Define the question and decision

Begin by stating what you need to know and what decision will follow from each possible result. A test may be used for screening, triage, diagnosis, or monitoring; clarify which purpose applies and which condition is under consideration.

A test has limited value if its result will not change management, follow-up, or a meaningful discussion with the patient. Diagnostic accuracy alone does not establish that testing improves outcomes. Questions might include whether pneumonia is likely enough to justify chest imaging, whether a blood test is needed to assess organ dysfunction, whether a culture could change antibiotic selection, or whether urgent imaging is needed to assess a possible intracranial bleed.

Balance urgency, benefit, and harm

Choose the timing and type of investigation according to how quickly information is needed and what is at stake. If a patient is unstable or a time-critical condition is suspected, begin stabilization and necessary treatment promptly. Obtain tests in parallel when feasible rather than allowing testing to delay urgent care.

A rapid bedside test may answer an urgent question, but it may be less definitive than confirmatory testing. Consider whether the result could change treatment, disposition, monitoring, or the need for further investigation, and whether it will arrive in time to guide the decision.

Consider patient-specific risks, including pregnancy, kidney function, allergies, comorbidities, and ability to tolerate a procedure. Also consider whether a false result or incidental finding could lead to repeat testing, invasive procedures, anxiety, or treatment that is not beneficial. Compare alternatives, including a safer, faster, less burdensome, or more informative test, and consider whether observation with planned reassessment is reasonable.

Discuss material risks, benefits, and alternatives with the patient and incorporate their preferences when time and circumstances allow.

Use probability to choose tests

Estimate the pretest probability of the suspected condition from the patient’s history, examination, risk factors, and relevant background rates. Then consider how much the result is likely to change that probability and whether the change would cross a threshold for action.

Sensitivity describes how often a test is positive when the condition is present. A sensitive test may be helpful when a negative result is intended to make the condition less likely. Specificity describes how often a test is negative when the condition is absent; a specific test may be helpful when a positive result is intended to make the condition more likely. Neither property makes a result definitive. Interpretation also depends on pretest probability, test timing, specimen quality, and the population in which the test is used.

When a condition is uncommon in the tested population, even a reasonably accurate test may produce more false positives than true positives. For example, among $1{,}000$ people with a $1\%$ chance of a condition, a test with $90\%$ sensitivity and $90\%$ specificity would produce about $9$ positive results among the $10$ people with the condition, and about $99$ positive results among the $990$ people without it. Only about $9$ of the $108$ positive results—roughly $8\%$—would be true positives. A positive result may therefore need confirmation, especially when the initial probability was low.

Avoid indiscriminate test panels and repeated testing without a clear question: each additional test can produce an unexpected or false-positive result and prompt further investigation. Conversely, do not use a reassuring result to dismiss strong clinical concern when the test was poorly timed, technically limited, or unable to exclude the dangerous diagnosis.

Interpret results and follow through

Interpret each result alongside the patient’s presentation, the test’s intended use, and its limitations—not in isolation. Check whether the result is technically valid, whether the sample was collected at an appropriate time, and whether the test measures the condition being considered.

A reference range is not automatically a diagnostic threshold, and a result outside the range does not by itself establish disease. Ask what a positive, negative, or indeterminate result means for this patient. Further testing may be needed to confirm a finding, resolve a mismatch between the result and the clinical picture, or identify an alternative diagnosis.

As results return, reassess the patient, update the differential, and review the response to treatment. Decide whether to continue, change, or stop the plan. Ensure significant results are communicated and acted upon, with responsibility for review, explanation to the patient, and follow-up clearly assigned.

Apply a practical checklist

Before ordering an investigation, ask:

1. What specific question am I trying to answer?

2. What will I do if the result is positive, negative, or unclear?

3. How urgent is the answer, and can testing occur without delaying stabilization or treatment?

4. How will the result change the probability of disease and the next decision?

5. What are the test’s limitations, risks, and likely downstream effects?

6. Who will review the result, explain it to the patient, and arrange follow-up?

Select investigations to answer a defined clinical question and inform a meaningful decision. Match the test and its timing to the patient’s urgency, pretest probability, and risks. Treat results as evidence rather than stand-alone answers, then reassess the patient and ensure findings lead to appropriate action.

• false positive